University of Illinois

No. 16/July 11, 1997

Current Status of European Corn Borers

Reports during the first full week of July and the early part of the second week confirmed our assessment of European corn borer infestations in Illinois. In some areas of the state, infestations were quite heavy; and, in other areas, corn borer activity was minimal. The outbreaks were localized, not widespread as they were in 1996. However, some consultants, dealers, and growers dealt with some severe infestations. Bill Craig, a private consultant in Macoupin County, indicated that infestations of first-generation European corn borers in Macoupin County and parts of Montgomery County were the heaviest he had observed in 20 years. John Thieme, with Zeneca Ag Products, observed a field in Henry County in which 83 percent of the plants were infested with an average of 3.3 larvae per plant.

Most people now are finding that larvae (fourth instars) have already bored into the stalks or are just about ready to do so (third instars). Obviously, the time for treatments to control first-generation European corn borers has passed. Although you might still find some small larvae in the whorls, if most of them have tunneled into the stalks, it's too late to treat.

Now it's time to wait for the second generation to begin. We have no idea what we will face; but in areas where the first generation was impressive, keep your eyes open. If mortality factors (weather, natural enemies) don't reduce the numbers of borers that complete the first generation, the second generation likely will be larger. Also stay alert in areas that escaped the first-generation problems. Again, good survival of corn borer adults, eggs, and early instar larvae will increase the potential for economic infestations of second-generation borers.

Ron Hines, a senior research specialist at the University of Illinois Dixon Springs Agricultural Center in "deep" southern Illinois, was splitting stalks to evaluate a Bt-corn trial during the first week of July; and he observed newly hatched European corn borer larvae. And they are catching European corn borer moths in their light traps. Apparently the second generation of these corn borers is under way, so we will have little time to relax between the two generations throughout the state. Table 1 shows some information regarding accumulated degree-days (developmental threshold of 50 degrees F) from initial capture of moths in the spring to first occurrence of life stage or activity of first- and second-generation European corn borers. If you are charting degree-day accumulations, this information should provide a "biofix" for your area and a way to keep track of the speed of development of the second generation of European corn borers.

Table 1. Accumulated degree-days (developmental threshold of 50 degrees F) from initial capture of moths in the spring to first occurrence of life stages and general activity of European corn borers (from European Corn Borer: Ecology and Management, NCR Publication no. 327, Iowa State University, Ames).

Accumulated degree-daysFirst occurrence of stage or eventDays to first occurrenceaGeneral activity
First generation
0First spring moth
212Egg hatch (first instar)16.3Pinhole leaf feeding
318Second instar6.6Shot-hole leaf feeding
435Third instar6.5Midrib and stalk boring
567Fourth instar6.6Stalk boring
792Fifth instar10.2Stalk boring
1,002Pupa7.6Changing to adult
Second generation
1,404Egg hatch (first instar)8.2Pollen and leaf axil feeding
1,510Second instar4.1Leaf axil feeding
1,627Third instar4.3Sheath and collar feeding, midrib boring
1,759Fourth instar5.1Stalk boring
1,984Fifth instar9.0Stalk boring

aAverage number of days of development to reach the first occurrence of the stage or event since initiation of the previous stage listed. For example, it takes approximately 16 days from first moth capture to egg hatch; first instars require approximately 6.6 days to develop to second instars; etc. The number of days varies if temperatures are cooler or warmer than average.

Kevin Steffey, Extension Entomology, (217)333-6652